Structure and fabricating method with self-aligned bit line contact to word line in split gate flash

ABSTRACT

A new structure is disclosed for semiconductor devices in which contact regions are self-aligned to conductive lines. Openings to a gate oxide layer, in partially fabricated devices on a silicon substrate, have insulating sidewalls. First polysilicon lines disposed against the insulating sidewalls extend from below the top of the openings to the gate oxide layer. Oxide layers are grown over the top and exposed sides of the first polysilicon lines serving to insulate the first polysilicon lines. Polysilicon contact regions are disposed directly over and connect to silicon substrate regions through openings in the gate oxide layer and fill the available volume of the openings. Second polysilicon lines connect to the contact regions and are disposed over the oxide layers grown on the first polysilicon lines.

This application is a continuation of U.S. application patentapplication Ser. No. 10/224,215, filed Aug. 20, 2002, now U.S. Pat. No.6,858,494.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates generally to semiconductor integratedcircuit technology and more particularly to split gate memory cells usedin flash EPROMs (Electrically Erasable Programmable Read Only Memory).

(2) Description of Prior Art

Increased performance in computers is often directly related to a higherlevel of circuit integration. Tolerances play an important role in theability to shrink dimensions on a chip. Self-alignment of components ina device serves to reduce tolerances and thus improve the packingdensity of chips. Other techniques can be important in shrinking devicesize. A method is disclosed later in the embodiments of the presentinvention of forming a structure with self-aligned bit line contact toword line through which a significant reduction in the area of the splitgate flash cell is possible.

As is well known in the art, split gate flash cells have bit lines andword lines and bit contacts that connect bit lines to drain regions. Bitlines and bit contacts are insulated from the word lines by aninterlevel dielectric layer. The separation between bit contacts andword lines must be maintained large enough so as to avoid possibleshorts that could develop between adjacent bit contacts and word lines.Bit contact to word line separations are determined by the positions ofbit contact openings, which are set by a design rule. In arriving at thedesign rule the possibility of misalignment must be taken into account,which results in a required separation well beyond that needed to avoiddevelopment of shorts. This requirement for increased separation,arising from the need to account for unavoidable misalignment, limitsthe ability to decrease cell size. Self-alignment of the bit contact tothe word line, as in the structures disclosed by the present invention,eliminates the reliability issue, allows a reduction in cell area andfacilitates shrinking the cell size.

A traditional method of fabricating a split gate flash memory cell ispresented in FIGS. 1 a–1 g, where top views of the cell are presented atsuccessive stages of the process and in FIGS. 2 a–2 g, which show thecorresponding cross-sections. A floating gate oxide, 6, is formed on asemiconductor substrate, 2, which preferably is a silicon substrate, toa thickness of about 80 Angstroms, followed by deposition of a poly 1layer, 8, to a depth of about 800 Angstroms. Active regions, 10, aredefined using isolating regions, such as shallow trench isolationregions, 4. This is followed by deposition of a nitride layer, whichpreferably is a silicon nitride layer to a depth of about 2500Angstroms. A photoresist layer, 14, is then formed as shown in FIGS. 1 band 2 b. The photoresist pattern, 14, is used in etching the siliconnitride layer to achieve the shape of region 12 of FIG. 2 b. It isadvantages to perform a poly 1 etch so as to achieve the shape of region8 as shown in FIG. 2 b. Details of the method to fabricate such sharppoly tips are presented in U.S. Pat. No. 6,090,668 to Lin et al., whichis herein incorporated by reference. Such sharp poly tips areadvantageous because they provide enhanced erase speed. After removal ofthe photoresist, an oxide 2 layer, 16, is deposited to a thickness ofabout 3000 Angstroms and a CMP (chemical-mechanical polishing) step isperformed. A second photoresist layer, 18, is formed and used insuccessively etching the silicon nitride layer and the poly 1 layer toachieve the structure shown in FIGS. 1 c and 2 c. Source regions 20 areformed by a P ion implantation at energy of about 20 keV and to a doseof about 4E14 per cm2. Removal of the second photoresist layer isfollowed by deposition of an oxide 3 layer to a depth of about 500Angstroms, which enhances the lateral diffusion of the source implant.An oxide 3 etching step is performed to achieve oxide 3 spacers, 22. Apolysilicon deposition is performed to a depth of about 3000 Angstromsand a CPM step on this layer produces a poly 2 region 24, which servesto contact the source 20. At this stage the structure is as depicted inFIGS. 1 d and 2 d. The traditional method proceeds with oxidation ofpoly 2, 24, to form about 200 Angstroms of oxide 4, 26. Next the nitridelayer 12 is removed, and successive etches are performed of the poly 1layer, 8, and floating gate oxide 1 layer, 6. After a poly 3 deposition,30, to about 2000 Angstroms, the structure is as shown in FIGS. 1 e and2 e. Etching the poly 3 layer, poly spacers, 30, are formed that serveas word lines. A drain implant is now performed that usually is an Asimplant at energy about 60 keV and to a dose of about 4E15 per cm2. Thisforms the drain regions 36. An interlevel dielectric (ILD) layer, 38 isdeposited. A photoresist layer is formed and patterned so that uponetching of the IDL layer, contacts are opened to the drain regions. Ametal 1 deposition follows removal of the photoresist layer. Anotherphotoresist layer is formed and patterned so that after etching metal 1bit lines 34 are formed connecting to the drain regions, 36 through themetal 1 contact regions 32. This completes the formation of atraditional split gate flash cell, which is shown in FIGS. 1 g and 2 g.

Bit lines, 34 and bit contacts, 32 are insulated from the word lines, 30by an interlevel dielectric layer, 38. The minimum separation, 40, isbetween bit contacts and word lines and this separation must bemaintained large enough so as to avoid possible shorts that coulddevelop between adjacent bit contacts and word lines. Bit contact toword line separations are determined by the positions of bit contactopenings relative to word lines and the dimensions of the openings,which are set by design rules. In arriving at the design rule thepossibility of misalignment and variability in the production of contactopenings must be taken into account, which results in a required minimumseparation well beyond that needed to avoid development of shorts. Thisrequirement for increased separation limits the ability to decrease cellsize. Self-alignment of the bit contact to the word line, as in thestructures disclosed by the present invention, eliminates thereliability issue, allows a reduction in cell area and facilitatesslinking the cell size.

A split-gate flash memory cell having self-aligned source and floatinggate self aligned to control gate, is disclosed in U.S. Pat. No.6,228,695 to Hsieh et al. In U.S. Pat. No. 6,211,012 to Lee et al. thereis disclosed an ETOX flash memory cell utilizing self aligned processesfor forming source lines and landing pads to drain regions. In U.S. Pat.No. 5,679,591 to Lin et al. there is disclosed a raised-bitlinecontactless flash memory cell. A method for fabricating a split-gateEPROM cell utilizing stacked etch techniques is provided in U.S. Pat.No. 5,091,327 to Bergemont.

SUMMARY OF THE INVENTION

It is a primary objective of the invention to provide a split gate flashcell with self-aligned bit contact to word line. It is also a primaryobjective of the invention to provide a method of forming a split gateflash cell with self-aligned bit contact to word line through which asignificant reduction in the split-gate flash cell area is possible.**As is well known in the art, a split-gate flash memory cell normally hassource and drain regions that are contacted by utilizing poly plugs.Insulating layers are required as spacers to separate these poly plugsfrom the floating gates and control gates of the cell, and this uses uparea. Furthermore, because of the high voltages required in the eraseoperation the spacer width cannot be decreased without paying a penaltyin reduced reliability. Elimination of the poly plugs, as in the methoddisclosed by the present invention, eliminates the reliability issue,allows a reduction in cell area and facilitates shrinking the cell size.Instead of poly plugs, a new self-aligned source/drain oxide etchingprocedure enables the formation of source/drain regions that areconnected in rows directly within the silicon. This procedure ofconnecting source/drains is generally applicable to arrays ofMOSFET-like devices.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawing forming a material part of this description,there is shown:

FIGS. 1 a–1 g show top views depicting a traditional method of formingsplit gate flash memory cells.

FIGS. 2 a–2 g show cross sectional views depicting a traditional methodof forming split gate flash memory cells.

FIGS. 3 a–3 j show top views depicting a method of forming split gateflash memory cells according to the invention.

FIGS. 4 a–4 j show cross sectional views depicting a method of formingsplit gate flash memory cells according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention are well described with the aidof FIGS. 3 a–3 j and 4 a–4 j. A method of fabricating a novel split gateflash memory cell is presented in FIGS. 3 a–3 j, where top views of thecell are presented at successive stages of the process and in FIGS. 4a–4 j which show the corresponding cross-sections. A floating gateoxide, 6, is formed on a semiconductor substrate, 2, which preferably isa silicon substrate, to a thickness of about 80 Angstroms, followed bydeposition of a poly 1 layer, 8, to a depth of about 800 Angstroms.Active regions, 10, are defined using isolating regions, such as shallowtrench isolation regions, 4. This is followed by deposition of a nitridelayer, which preferably is a silicon nitride layer to a depth of about2500 Angstroms. A photoresist layer, 14, is then formed as shown inFIGS. 3 b and 4 b. The photoresist pattern, 14, is used in etching thesilicon nitride layer to achieve the shape of region 12 of FIG. 4 b. Apoly 1 etch is performed, and it is preferred to achieve the shape ofregion 8 as shown in FIG. 4 b according to the method described in U.S.Pat. No. 6,090,668 to Lin et al., which is herein incorporated byreference. Such sloped segments of the poly 1 layer provide improvedoperation of the memory cell. After removal of the photoresist, an oxide2 layer, 16, is deposited to a thickness of about 3000 Angstroms and aCMP (chemical-mechanical polishing) step is performed. A secondphotoresist layer 18, is formed and used in successively etching thesilicon nitride layer and the poly 1 layer to achieve the structureshown in FIGS. 3 c and 4 c. Source regions 20 are formed by a P ionimplantation at energy of about 20 keV and to a dose of about 4E14 percm2. Removal of the second photoresist layer is followed by depositionof an oxide 3 layer to a depth of about 500 Angstroms, which enhancesthe lateral diffusion of the source implant. An oxide 3 etching step isperformed to achieve oxide 3 spacers, 22. A polysilicon deposition isperformed to a depth of about 3000 Angstroms and a CPM step on thislayer produces a poly 2 region 24, which serves to contact the source20. At this stage the structure is as depicted in FIGS. 3 d and 4 d. Themethod proceeds with oxidation of poly 2, region 24, to form about 200Angstroms of oxide 4, region 26. Next the nitride layer 12 is removed,and successive etches are performed of the poly 1 layer, 8, and floatinggate oxide 1 layer, 6. An oxide 5 layer, 28, is then depositedconformally covering the surface of the substrate 2 and the protrudingprofile of the oxides 16 and 26. A poly 3 layer, 30, is then conformallydeposited on the oxide layer 28 to about 2000 Angstroms. The resultingstructure is as shown in FIGS. 3 e and 4 e. At this point the method ofthe invention deviates from traditional methods. Instead of immediatelyperforming an etch back step to form poly 3 spacers 30, as intraditional methods, in which a rounded shape results, in the method ofthe invention a CMP process step is inserted before the poly 3 etchback. After the poly CMP step a more square profile is achieved for thepoly 3, 42. As a result an essentially vertical profile is achieved forthe poly 3 spacers 44, which are formed by etching hack the poly 3region, 42. The oxide 5 layer 28 remaining on the drain area is nowremoved, which can be accomplished by a wet dip oxide etch or by anoxide dry etch. There follows an oxidation step in which oxide 6, 46, isgrown to a thickness of about 600 Angstroms over the exposed poly 3. Anoxide of about half this thickness, 48, is grown, in this oxidationstep, to the undoped silicon region in the drain area, so that thethickness of the oxide in that region is about 300 Angstroms. Such adifference in thickness is due to the significantly reduced oxide growthrate of undoped silicon substrate as compared to doped poly. The oxidegrowth rate of doped poly is about twice that of undoped silicon. Thedifference in thickness of the oxides in regions 46 and 4 8, aconsequence of the difference in oxide growth rate, is important to theimplementation of the invention. The next step is to form drain regions52. This is preferably accomplished with an implant of As ions at energyof about 60 keV and to a dose of about 4E15 per cm2. All oxide spaceretch follows in which all the oxide 48 over the drain region is etchedaway, but oxide 6 layers over poly 3, 50 will remain, however at areduced thickness of about 260 Angstroms. The remaining oxide 6 layer 50serves as an insulating layer for the underlying poly 3 spacers, whichact as word lines. A square profile is preferred since more oxideremains, subsequent to the spacer oxide etch on the word line sidewallsfor a square profile. This allows for the direct deposition of a poly 4layer, which is performed to a depth of about 2000 Angstroms. Nointervening interlevel dielectric layer is required. Another photoresistlayer is formed and patterned so that after etching poly 4, bit lines 54are formed connecting to the drain regions, 52 through the poly 4contact regions 56. This completes the foliation of a split gate flashcell according to the invention, which is shown in FIGS. 3 j and 4 j.

Bit lines, 54 and bit contacts, 56 are insulated from the word lines, 30by an oxide 6 layer, 50 that was grown directly on the word lines and isof a thickness sufficient to reliably insulate the word lines from thebit lines and bit contacts. No area need be devoted to account formisalignment or imperfect accuracy in the dimension of these regions.Self-alignment of the bit line and bit contact to the word line, as inthe structure of the present invention, eliminates the reliabilityissue, allows a reduction in cell area and facilitates shrinking thecell size.

Other preferred embodiments of the invention are applicable tosituations where, in partially fabricated devices on a silicon substratethere are openings to a gate oxide layer disposed over the substrate.The openings are to contain a first conductive line disposed over theoxide and a contact region, connecting a second conductive line to thesilicon substrate that needs to be insulated from the first conductiveline. The second conductive line passes over the first conductive lineand needs to be insulated from the first conductive line. In the methodof the invention a first polysilicon layer is deposited to more thancover the openings. A CMP step is performed stopping at the top of theopenings. Etching back the first polysilicon layer follows to producepolysilicon spacers with essentially rectangular profiles over the gateoxide layer adjacent to the opening sidewalls and defining diminishedopenings to the gate oxide layer. An oxidation step is then performedthat results in an oxide layer grown over the exposed surfaces of thepolysilicon spacers. For a gate oxide layer about 170 Angstroms thickthe oxide over the polysilicon spacers should be grown to a thickness ofabout 600 Angstroms. Additional oxide is also grown during the oxidationstep, but to a lesser extent, under the exposed gate oxide layer in theopenings. The thickness of this layer is increased to about 340Angstrom, if the original gate oxide thickness was 170 Angstroms and 600Angstroms is grown on the polysilicon spacers. Only about 170 Angstromsis added mainly due to the significantly reduced oxide growth rate ofthe undoped silicon substrate as compared to doped polysilicon. Theoxide growth rate of doped poly is about twice that of undoped silicon.Also contributing to the relatively small increase in thickness is thatthe additional oxide is grown under the gate oxide layer that was thereprior to the oxidation step. Drain regions can now be formed ifrequired. This is preferably accomplished with an implant of As ions atenergy of about 60 keV and to a dose of about 4E15 per cm2. A spaceroxide etch follows in which all the oxide over the silicon substrate ofthe openings is etched away, but an oxide layer will remain over thepolysilicon spacer, however at a reduced thickness of about 260Angstroms. This remaining oxide layer serves as an insulating layer forthe underlying polysilicon spacers. A deposition of a second polysiliconlayer follows, which is preferably performed to a depth of about 2000Angstroms. No intervening interlevel dielectric layer is required. Thesecond polysilicon layer filling the openings serve as contact regions.A photoresist layer is formed and patterned so that after etching thesecond polysilicon conductive lines are formed connected to the siliconsubstrate through the contact regions.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.

1. A method of forming a structure for split gate flash memorycomprising: providing a semiconductor substrate comprising split gatestructures and drain surfaces, where a first insulator layer is formedover the split gate structures; forming doped polysilicon spacer regionsbetween the split gate structures and the drain surfaces; oxidizing thedrain surfaces to form a second insulator layer and oxidizing surfacesof the doped polysilicon spacer regions to form a third insulator layer,wherein the thickness of the second insulator layer is thinner than thethird insulator layer; removing the second insulator layer over thedrain surfaces; implanting ions into the drain surfaces to form drainregions; and forming a conductive layer over the drain regions.
 2. Amethod of forming a structure for split gate flash memory comprising:providing a semiconductor substrate; forming split gate structures overthe semiconductor substrate; conformably forming a first insulator layerover the split gate structures and the semiconductor substrate; formingconductive spacer regions over the first insulator layer; etching thefirst insulator layer to expose drain surfaces of the semiconductorsubstrate; forming a second insulator layer over the drain surfaces anda third insulator layer over the conductive spacer regions, wherein thethickness of the second insulator layer is thinner than the thirdinsulator layer; removing the second insulator layer over the drainsurfaces; implanting ions into the drain surfaces to form drain regions;and forming a conductive layer over the drain regions.
 3. A method offorming a structure for split gate flash memory comprising: providing asemiconductor substrate; forming split gate structures over thesemiconductor substrate; conformably forming a first insulator layerover the split gate structures and the semiconductor substrate; forminga first conductive layer over the first insulator layer; planarizing thefirst conductive layer to the top of the first insulating layer; etchingthe first conductive layer to form square conductive spacer regionsagainst sidewalls of the split gate structures that serve as word lines;etching the first insulator layer to expose drain surfaces of thesemiconductor substrate; forming a second insulator layer over the drainsurfaces and a third insulator layer over the square conductive spacerregions; removing the second insulator layer over the drain surfaces;forming drain regions in the drain surfaces; and forming a secondconductive layer over the drain regions that serve as bit lines.